Multisound reproducing apparatus for deriving four sound signals from two sound sources

ABSTRACT

Left and right sound signals applied to two separate input circuits are each shifted in phase by a predetermined amount by phase shifters and are then supplied to separate output circuits. The left sound signal is also fed through a low pass filter to be combined with the phase shifted right sound signal and this combined signal is supplied to a separate output circuit. Likewise the right sound signal is fed through a low pass filter to be combined with the phase shifted left sound signal and this combined signal is also supplied to a separate output circuit. The resultant four outputs give the listener the illusion of naturalness.

mite tates atet n 1 iida I45] Apr. 3, W73

[54] MULTESOUND REPRODUCKNG APPARATUS FOR DERIVING FOUR SOUND SIGNALSFROM TWO SOUND SOURCES Kazumi Iida, Kanagawa, Japan [30] ForeignApplication Priority Data Apr. 13, 1971 Japan ..46/23300 [52] US. Cl..l79/l G, 179/1 GP [51] Int. Cl. ..H04r 5/04 [58] Field of Search.....179/l G, 1 GP, 1 G0, 15 BT, 179/100.4 ST, 100.1 TD

[56] References Cited UNITED STATES PATENTS 8/1972 Orban ..l79/1 GPrimary ExaminerKathleen H. Claffy Assistant Examiner-Thomas DAmicoAttorney-Lewis H. Eslinger et al.

[57] ABSTRACT Left and right sound signals applied to two separate inputcircuits are each shifted in phase by a predetermined amount by phaseshifters and are then supplied to separate output circuits. The leftsound signal is also fed through a low pass filter to be combined withthe phase shifted right sound signal and this combined signal issupplied to a separate output circuit. Likewise the right sound signalis fed through a low pass filter to be combined with the phase shiftedleft sound signal and this combined signal is also supplied to aseparate output circuit. The resultant four outputs give the listenerthe illusion of naturalness.

8 Claims, 11 Drawing Figures FILTER MULTISOUND REPRODUCING APPARATUS FORDERIVING FOUR SOUND SIGNALS FROM TWO SOUND SOURCES BACKGROUND OF THEINVENTION This invention relates to a multisound decoding apparatus andmore particularly to apparatus for reproducing a monaural audioinformation signal or stereophonic audio information signal with fourseparate loudspeakers without accompanying unnaturalness.

There is increasing interest in multiple channel recording andreproducing systems because of the variety of sounds and music formswhich such systems reproduce. It is a well known phenomenon that thequality of music reproduction is enhanced when the number ofreproduction channels is increased. In the early days of the phonograph,only a single channel or monophonic recording was used. Investigatorssoon realized the value, however, of recording and transmitting twoseparate channels of information, which in modern parlance is known asbinaural or stereophonic sound.

However, even two channels of information are not considered sufficientfor a'full illusion of reality. For example, when a listener is placedin front of a symphony orchestra, he hears sounds arriving from manydifferent directions and from a variety of instruments, as well asreflections from the walls andceiling, which gives him an accustomedillusion of space perspective. When reproduction is accomplished byutilizing only two channels, however, it is difficult if not impossible,to produce true reality with respect to spatial perspective. Earlyexperiments have demonstrated that a minimum of three independentchannels are needed to convey a satisfactory illusion of reality in thereproduction of orchestral music.

Recently, some investigators have developed a multichannel sound system,so called a four channel stereo system, providing third and fourthplayback channels to an otherwise two-channel system by feeding thirdand fourth loudspeakers with signals.

If suitable sound media are prepared for the above mentionedmultichannel sound system having the third and fourth channels,listeners may hear the sound from the sound system with a full illusionof reality. At present, however, sound media which can be used for suchmultichannel sound systems are insufficient and stereophonic sound mediafor two channels command a prominent majority.

In an attempt to provide a four channel system for use with stereophonicsound media one proposed prior system separates a stereophonic soundinformation signal into left and right sound signals defined as L and R,respectively, by a channel separator. The L signal is supplied to aloudspeaker placed on the left-hand side of a listener and in front ofhim while the R signal is supplied to a loudspeaker positioned on theright-hand side of the listener and in front of him as in a conventionalmanner. The proposed system further inverts the phase of the L signal tomake it a signal L and supplies it to a loudspeaker positioned on theleft-hand side of the listener at his back. Similarly the R signal isinverted in phase to be R and is supplied to a loudspeaker positioned onthe right-hand side of the listener at his back.

In other prior systems the signal R is supplied to the left rearloudspeaker while the signal L is supplied to the right rearloudspeaker. In still other prior systems a matrix circuit is employedfor applying signals L-R and R-L to the loudspeakers at the left-handand right-hand rear sides of the listener, respectively.

With such conventional systems, since the signals having inverted phasecomponents are supplied to the loudspeakers at the back of the listener,he generally hears uncomfortable sounds peculiar to the inverted phasecomponents when they are reproduced. Although there are provided the tworear loudspeakers in addition to the two front loudspeakers so that thesocalled sound field is widened there still occurs a drawback in thatthe separation ability for the rear channels is greatly deteriorated.

SUMMARY OF THE INVENTION The above and other disadvantages are overcomeby the present invention of a multisound reproducing system comprising afirst and a second input circuits for receiving a first and a secondsignal, respectively, means connected to the first and the second inputcircuits for separately shifting the phase of the first and the secondsound signals by a predetermined amount, means responsive to the phaseshifted first and second sound signals for separately reproducing them,means for filtering the first sound signal and for combining it with thephase shifted second sound signal to produce a third sound signal, meansfor filtering the second sound signal and for combining it with thephase shifted first sound signal to produce a fourth sound signal, andmeans responsive to the third and the fourth sound signals forseparately reproducing them.

In one embodiment when the first and the second sound signals initiallyhave the same phase and are at a predetermined frequency, the phaseshifting means is designed to shift their phases by one hundred andeighty degrees with respect to their phases after being filtered by eachof the combining means.

In another embodiment the system is provided with switches and anadditional set of phase shifting circuits connected to the first and thesecond input circuits forproviding of additional phase shift over theprimary phase shifting means. The switches allow the listener toselectively and separately reproduce the third and the fourth soundsignals or the output signals from the additional phase shiftingcircuits, thereby making the system compatible with either stereophonicor quadraphonic sound media, respectively.

Accordingly, one object of the invention is to provide a multisoundreproducing system in which four independent signals are produced fromstereophonic sound signals supplied by two channels and the fourindependent signals are supplied to at least four loudspeakers.

Another object of the invention is to provide a multisound reproducingsystem for reproducing a sound signal about the circumference of thelistener without discomfort to the listener.

A further object of the invention is to provide a multisound reproducingsystem in which a monaural signal is reproduced as a monaural soundpositioned in front of a listener.

A still further object of the invention is to provide a multisoundreproducing system which allows the sound to be positioned in accordancewith the frequency which is most sensitive for the listener.

Yet another object of the invention is to provide a multisoundreproducing system which is compatible with both stereophonic andfour-channel reproducing systems.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing oneembodiment of the invention;

FIGS. 2A, 2B and 2C are graphs for explaining the operation of theembodiment depicted in FIG. 1;

FIG. 3 is a plane view of the loudspeaker arrangement of the embodimentof FIG. 1;

FIGS. 4A, 4B, 5A and 5B are plane views of the reproduced sound fieldsfor different input signals to the embodiment of FIG. 1,diagrammatically illustrating the directions of the sounds reproduced bythe embodiment of FIG. 1;

FIG. 6 is a block diagram of another embodiment of the invention; and

FIG. 7 is a circuit diagram of a part of the embodiment depicted in FIG.6.

DESCRIPTION OF CERTAIN PREFERRED EM BODIMENTS Referring now moreparticularly to FIG. 1, reference numeral 10 denotes a stereophonicsignal source having two channels such as a tape recorder, recordplayer, FM multiplex receiver or the like. One of the channels of thestereophonic signal source supplies a signal L which is fed through anamplifier 51 to a phase shifter circuit 61 which produces as its outputa signal Lf. The output of the other channel of the stereophonic signalsource 10 is a signal R which is fed through an amplifier 52 to a phaseshifter circuit 62 which produces as its output a signal Rf. The outputsignals Lf and Rf from the phase shifter circuits 61 and 62 arerespectively supplied through power amplifiers 71 and 72 to loudspeakers1 and 2, respectively. The loudspeaker l is placed on the left-hand sideof the listener 5 in front of 5 him and the loudspeaker 2 is placed onthe right-hand side and in front of the listener as illustrated in FIG.3.

The phase versus frequency characteristics of the phase shifter circuits61 and 62 are illustrated in FIG. 2A and are designated 1116. Within apredetermined frequency range centered about a predetermined frequencyf6the phase shift for an incoming signal varies approximately in directproportion to the logarithm of its frequency from 0 of phase shift to360 of phase shift.

In particular, at the predetermined frequency f6, (which is, for exampleabout 400 Hz) the phase of an incoming signal is shifted I80". At theouter limits of the frequency range of the phase shifter circuits thereis no phase shifting (0 and 360 phase shifts). Thus the signals Lf andRf are inverted in phase when the signals L and R are at thefrequency f6and have the same phase as the signals L and R at the fringes of theband range.

The amplitude versus frequency characteristic of the phase shiftercircuits 6l'and 62 are illustrated by the curve A6 in the FIG. 2B. Asillustrated, the response of the phase shifter circuits is substantiallyflat over a broad range of audio frequencies.

The signal L from the amplifier 51 is also supplied to the input of afilter 84 which produces a signal La at its output. The signal R fromthe amplifier 52 is supplied to the input of a filter 83 which producesa signal Ra at its output. The filters 83 and 84 have amplitude versusfrequency characteristics designated A8 in the FIG. 2B. As illustratedin the Figure, the filters are both low pass filters having a 3db cutoffat a predetermined frequencyfB, which is, for example a frequencyslightly higher than 400 Hz, and they attenuate at -6db/octive.

The phase versus frequency characteristic of each filter is depicted bythe curve (118 in FIG. 2A. As illustrated in the Figure the outputsignals La and Ra are phase shifted from 0 to 90 in direct proportion tothe logarithm of the frequency of the input signals L and R within aband range centered about a frequency f8. At the predetermined frequencyf8 the signal La or Ra is shifted by 45 with respect to the signal L orR. At the lower end of the frequency range it has the same phase and atthe upper end of the frequency range it is phase shifted by 90 withrespect to the signal L or R.

The signals La and Rf are combined in an adder circuit 94 to produce acombined output signal Rb which is fed through an amplifier 74 to aloudspeaker 4. The signals Lf and Ra are combined in an adder circuit 93which produces as its output a signal Lb which is fed through a poweramplifier 73 to a loudspeaker 3. As illustrated in FIG. 3 theloudspeaker 3 is at the left rear side of the listener 5 and theloudspeaker 4 is at the right rear side of the listener 5.

In operation the signals Lf and Rf are both shifted in phase by equalamounts by the phase shifters61 and 62, respectively, and theiramplitudes are constant over the frequency range of the system.Therefore the signals L, R, Lf and Rf are equal with each other in leveland the phase difference between the signals Lf and Rf is equal to thephase difference between the signals L and R. It also follows that thesounds S1 and S2 are reproduced with the same phase and the same levelfrom the loudspeakersl and 2, respectively, as shown in FIG. 4A.

In the adder circuit 93 the signals Lf and Ra are added together toproduce the signal Lb. The phase versus frequency characteristics of thephase shifters 61 and 62 and the filters 83 and 84 are such that at apredetermined frequency f7 (which is, for example between 650 to 700 Hz)there is l difference in phase between signals emerging from either ofthe filters 83 and 84 and signals emerging from either of the phaseshifters 61 and 62. Thus at the predetermined frequency f7 the signalsLf and Ra are phase inverted with respect to each other and cancel.

Thus the signal Lb has no signal components near the frequency f7 asillustrated in FIG. 2C by dotted lines. The signal Ra delivered from thefilter 83 has an amplitude characteristic as illustrated by the curve A8in FIG. 2B and as a result the frequency spectrum of the signal Lb hasno signal-components near the frequency f7 as illustrated by the solidline curve in the FIG. 2C and is attenuated in its intermediate and highfrequency signal components. The main part of the signal Lb is a signalcomponent lower than the frequency f7. Since the signal L is equal inlevel and phase to the signal R, it

follows that the above considerations apply equally to the signal Rbwith the result that the signal Rb is equal to the signal Lb.

The signals Lb and Rb are respectively fed through the power amplifiers73 and 74 to be reproduced as sounds S3 and S4, respectively, from theloudspeakers 3 and 4 as illustrated in FIG. 4A. Since the maincomponents of the signals Lb and Rb are low frequency, the reproducedsounds S3 and S4 have almost no contribution to the position of thesound image as perceived by the listener 5. Therefore if the signals Land R are equal the position of the sound image as perceived by thelistener is midway between the loudspeakers l and 2.

In the situation where only the signal L exists (there is no signal R),then the sound S1 is reproduced from the loudspeaker 1 from the signalLf but there is no sound signal reproduced from the loudspeaker 2 asillustrated in FIG. 4B. The signal Lb derived from the adder circuit 93contains only the signal Lf which is then reproduced through theloudspeaker 3 as the sound S3 having the same level as the sound S1. Thesignal Rb from the adder circuit 94 contains only the signal La obtainedfrom the filter 84. The amplitude characteristic of the signal Rb isshown by the curve A8 in FIG. 2B, so that the signal Rb consistsprimarily of the low frequency signal component of the signal L and theattenuated intermediate frequency signal component thereof. The phase ofthe signal Rb is also substantially the same as the phase of the signalLf. The signal Rb is reproduced from the loudspeaker 4 as the sound S4.

Thus where only the signal L exists and the signal R does not exist, thereproduced sound S1 from the loudspeaker I and $3 from the loudspeaker 3are the same in phase and level and the reproduced sound S4 from theloudspeaker 4 contains only the low frequency signal component and theattenuated intermediate frequency signal component of the signal L. Thesound image as perceived by the listener 5 is positioned near theloudspeaker 3. The separation for left and right side sounds iseffectively kept because the sound S4 contains no high frequency signalcomponent of the signal L and its intermediate frequency signalcomponent is substantially attenuated.

Similarly in the case where only the signal R exists, and no signal Lexists, the sound image is positioned near the loudspeaker 4 asperceived by the listener 5.

The above examples of sound positioning by the reproducing system of theinvention indicate that although the signals L and R are normallyreproduced by a two channel stereophonic reproducing system as a soundimage S evenly distributed in front of the listener (FIG. 5A) the samesignals when reproduced by the embodiment of FIG. 1 provide a soundimage S which is expanded to surround the listener from his left rearside, around in front of him, to his right rear side (FIG. 5B). Thisincreases the illusion of reality for the sound effect. For example thesystem of the invention is capable of giving'the illusion of a singerstanding directly in front of the listener 5 and with musicalinstruments being played around the listener.

It is presently considered that a man can hear a sound having afrequency of approximately 700 Hz with best sensitivity. This is thefrequency f! as discussed above. Therefore in the situation where thesignals L and R have the same phase the loudspeakers 3 and 4 behind thelistener 5 produce no sounds having a 'more particularly to FIG. 6 aquadraphonic stereo reproducing system according to the invention has afirst input terminal 101 a second input terminal 102 and a first,second, third and fourth output terminals 103, 104, 105 and 106,respectively. A first and a second phase shifter 107 and 108 areconnected to the first input terminal 101 and a third and a fourth phaseshifter 109 and 110 are connected to the second input terminal 102. Thephase shifter 108 provides a phase difference of 90 with respect to thephase shifter 107 and a phase shifter 109 provides a phase difference of90 with respect to the phase shifter l 10.

Quadraphonic signals supplied to the first and second input terminals101 and 102 are encoded to comprise signals LT and RT from, for example,four original sound informational signals LF, RF, LB and RB. Thecomposite signal LT consists of three components LF, 0.7LB and 0.7RB.The signals LF and 0.7RB have the same phase whereas the signal 0.7LB is270 different in phase from the signal LP. The other composite signal RTsimilarly consists of three components RF, 0.7RB and 0.7LB. The signal0.7LB is 180 out of phase with the signal RF and the signal 0.7RB is 90out of phase with the signal RF.

The composite signal LT applied to the input terminal 101 is suppliedthrough a first phase shifter 107 and a power amplifier 114 to theoutput terminal 103 from which a signal LP is derived. A loudspeaker(not shown) positioned at the left front side of the listener isconnected to the output terminal 103. The loudspeaker reproduces a soundhaving the signal LT as its main component. The composite signal RTapplied to the second input terminal 102 is passed through phase shifter110 and the power amplifier 116 to emerge as the signal RF from thefourth output terminal 106. A

loudspeaker (not shown) is connected to the output terminal 106 and isplaced at the right front side of the listener to reproduce a soundhaving the signal RF as its main component.

The composite signal LT supplied to the first input terminal 101 is alsofed to the second phase shifter 108 where is is shifted in phase 90 withrespect to the output of the first phase shifter 107. The phase-shiftedcomposite signal from the phase shifter 108 is supplied through aresistor 120 to an adder circuit 121 where it is combined with theoutput from the phase shifter 110 supplied to the adder 121 through aresistor 122.

The output from the adder circuit 121 is fed through a phase inverter123 to a fixed contact b of a single pole, double throw switch 124. Themoving contact c of the switch 124 is connected to the input of a poweramplifier 126" whose output is supplied to the output terminal 104. Thesignal LB derived at the output terminal 104 has the components LB 0.7LF0.7RF. It has the same phase as the signal LB but is 90 out of phasewith the signal 0.7LF and is out of phase with the signal 0.7RF. Theoutput terminal 104 is connected to a loudspeaker (not shown) placed atthe rear right hand side of the listener.

The composite signal RT is also supplied to the phase shifter 109 whereit is phase shifted by 90 with respect to the output signal from thephase shifter 110. The phase-shifted composite signal from the phaseshifter 109 is supplied through a resistor 130 to an adder circuit 131where it is combined with the output signal from the phase shifter 107which is supplied to the adder 131 through a resistor 132. The outputsignal from the adder circuit 131 is supplied directly to a fixedterminal b of a single pole, double throw switch 134. The contact arm cof the switch 134 is connected to the input of a power amplifier 136which has its output connected to the output terminal 105. The signal RBderived at the output terminal 105 contains the signal components RB0.7LF 0.7RF. It has the same phase as the signals RB and 0.7LF but is270 out of phase with the signal 0.7RF. The terminal 105 is connected toa loudspeaker (not shown) which is positioned at the rear right handside of the listener.

In order to make the above described system compatible with bothconventional stereophonic signals and quadraphonic signals the switches124 and 134 each have two fixed contacts a and b and a movable contactc. The fixed contact a of each switch serves to transmit a conventionalstereophonic signal to the respective output terminals 104 and 105 andthe other fixed contact b serves to transmit quadraphonic signals to therespective output terminals 104 and 105.

The input side of a first low pass filter 151 is connected to the inputterminal 101 and its output side is connected to an adder circuit 154through a resistor 153. The input side of a second low pass filter 152is connected to the input terminal 102 and its output side is connectedto an adder circuit 156 through a resistor 155.

The output side of the phase shifter circuit 110 is connected through aresistor 157 to the adder circuit 154. The signals from the low passfilter 151 and the phase shifter circuit 110 are combined by the adderand its output signal is connected to the terminal a of the switch 134.The output side of the phase shifter 107 is connected to the addercircuit 156 through a resistor 158. The signals from the low pass filter152 and from the phase shifter circuit 107 are combined in the addercircuit 157 and its output is connected to the fixed terminal a of theswitch 124.

If the movable contacts c, which are ganged together, of the twoswitches 124 and 134 are connected to their respective fixed contacts athen the stereophonic signals L and R supplied to the input terminals101 and 102 are reproduced as the signals LF and RF at the outputterminals 103 and 106 while the respective signals LB and RB are derivedat the output terminals 104 and 105 to achieve an effect similar to thatdescribed with respect to the embodiment of FIG. 1. It should be notedthat in this mode of operation the phase shifters 107 and 110 areemployed.

As described above when the movable contacts c of the switches 124 and134 are connected to the fixed contacts b and the quadraphonic signalsLT and RT are supplied to the input terminals 101 and 102, respectively,then the signal LP is derived at the output terminal 103, the signal LBis derived at the output terminal 104, the signal RB is derived at theoutput terminal 105 and the signal RF is derived at the output terminal106.

Referring now more particularly to H0. 7 some of the features of theembodiment of FIG. 6 are described in more detail. The first phaseshifter 107 comprises two transistors 107A and 1078. A series circuit ofa capacitor 1070 and a resistor 107d is connected between the emitterand collector electrodes of the transistor 107A. The connection pointbetween the capacitor 1070 and the resistor 107d is connected to thebase of the transistor 1073. A series circuit of a capacitor 107e and aresistor 107f is connected between the collector and emitter electrodesof the transistor 107B. The connection point between the capacitor 107eand the resistor 107f is connected to the output terminal 103 through aresistor and a power amplifier l 14.

The other phase shifting circuits 108, 109 and 110 are similarlyconstructed and an explanation of their construction is omitted for thesake of simplicity. Since the phase shifters 108 and 109 are used forphase shifting signals by with respect to the signals passed through thephase shifters 107 and 110, the value of the capacitors and resistorsconnected between the leads of the transistors of the phase shifters 108and 109 are selected to be different from those of the capacitors andthe resistors connected to the transistors of the phase shifters 107 and110.

The low pass filter 151 comprises a resistor 151R connected between theinput terminal 101 and one lead of a capacitor 151C. The other lead ofthe capaci tor 151C is connected to the circuit ground. The output fromthe low pass filter 151 is taken from the connection point between theresistor 151R and the capacitor 151C. Similarly the low pass filter 152comprises a resistor 152R connected between the input terminal 102 andone lead of a capacitor 152C. The other lead of a capacitor 152C isconnected to the circuit ground. The outputfrom the low pass filter 152is derived from the connection point between the resistor 152R and thecapacitor 152C.

The terms and expressions which have been em ployed here are used asterms of description and not of limitation, and there is no intention inthe use of such terms and expressions, of excluding equivalents of thefeatures shown and described, or portions thereof, it being recognizedthat various modifications are possible within the scope of theinvention claimed.

What is claimed is:

1. A multisound reproducing system comprising a first input circuit forreceiving a first sound signal, a second input circuit for receiving asecond sound signal, a first phase shifting circuit connected to thefirst input circuit for shifting the phase of the first sound signal bya predetermined phase angle, a second phase shifting circuit connectedto the second input circuit for shifting the phase of the second inputsignal by a predetermined phase angle, means responsive to the phaseshifted first input signal for producing a first output, meansresponsive to the phase shifted'second input signal for producing asecond output, a first filter circuit for filtering the first inputsignal, a second filter circuit for filtering the second input signal,means for combining the filtered first input signal with the phaseshifted second input signal and for producing a third signal, means forcombining the filtered second input signal with the phase shifted Ifirst input signal to produce a fourth signal, means responsive to thethird signal for producing a third output and means responsive to thefourth signal for producing a fourth output.

2. A multisound reproducing system as recited in claim 1 wherein each ofthe filtering circuits comprises a low pass filter.

3. A multisound reproducing system as recited in claim 1 wherein thephase shift provided by each of the first and the second phase shiftingcircuits varies in substantially direct proportion to the logarithm ofthe frequency of the signal being phase shifted within a band rangecentered about a predetermined frequency, the phase shift provided atthe predetermined frequency being 180.

4. A multisound reproducing system as recited in claim 1 wherein thefirst and the second filter circuits and the first and the second phaseshifting circuits are designed such that when the first and the secondinput signals have the same phase and are at a predetermined frequencythe output signals from the first and the second filter circuits eachhave a phase difference of 180 with respect to the outputs from thefirst and the second phase shifting circuits.

5. A multisound reproducing system as recited in claim 4 wherein thepredetermined frequency is selected to be within the range of 650 to 700Hz.

6. A multisound reproducing system as recited in claim 1 furthercomprising a third phase shifting circuit connected to the first inputcircuit for shifting the phase of the first input signal by apredetermined angle with respect to the output signal from the firstphase shifting circuit and a fourth phase shifting circuit connected tothe second input circuit for shifting the phase of the second inputsignal by a predetermined angle with respect to the output signal fromthe second phase shifting circuit.

7. A multisound reproducing system as recited in claim 6 wherein thepredetermined angle of phase shift provided by the third and the fourthshifting circuits is 8. A multisound reproducing system as recited inclaim 6 further comprising switching means connected to the outputs ofthe third and the fourth phase shifting circuits and to the outputs ofthe means for producing the third signal and the means for producing thefourth signal for selectively transmitting the third and the fourthsignals or the phase shifted signals from the third and the fourth phaseshifting circuits to the means for producing the third and the fourthoutputs.

1. A multisound reproducing system comprising a first input circuit forreceiving a first sound signal, a second input cIrcuit for receiving asecond sound signal, a first phase shifting circuit connected to thefirst input circuit for shifting the phase of the first sound signal bya predetermined phase angle, a second phase shifting circuit connectedto the second input circuit for shifting the phase of the second inputsignal by a predetermined phase angle, means responsive to the phaseshifted first input signal for producing a first output, meansresponsive to the phase shifted second input signal for producing asecond output, a first filter circuit for filtering the first inputsignal, a second filter circuit for filtering the second input signal,means for combining the filtered first input signal with the phaseshifted second input signal and for producing a third signal, means forcombining the filtered second input signal with the phase shifted firstinput signal to produce a fourth signal, means responsive to the thirdsignal for producing a third output and means responsive to the fourthsignal for producing a fourth output.
 2. A multisound reproducing systemas recited in claim 1 wherein each of the filtering circuits comprises alow pass filter.
 3. A multisound reproducing system as recited in claim1 wherein the phase shift provided by each of the first and the secondphase shifting circuits varies in substantially direct proportion to thelogarithm of the frequency of the signal being phase shifted within aband range centered about a predetermined frequency, the phase shiftprovided at the predetermined frequency being 180* .
 4. A multisoundreproducing system as recited in claim 1 wherein the first and thesecond filter circuits and the first and the second phase shiftingcircuits are designed such that when the first and the second inputsignals have the same phase and are at a predetermined frequency theoutput signals from the first and the second filter circuits each have aphase difference of 180* with respect to the outputs from the first andthe second phase shifting circuits.
 5. A multisound reproducing systemas recited in claim 4 wherein the predetermined frequency is selected tobe within the range of 650 to 700 Hz.
 6. A multisound reproducing systemas recited in claim 1 further comprising a third phase shifting circuitconnected to the first input circuit for shifting the phase of the firstinput signal by a predetermined angle with respect to the output signalfrom the first phase shifting circuit and a fourth phase shiftingcircuit connected to the second input circuit for shifting the phase ofthe second input signal by a predetermined angle with respect to theoutput signal from the second phase shifting circuit.
 7. A multisoundreproducing system as recited in claim 6 wherein the predetermined angleof phase shift provided by the third and the fourth shifting circuits is90* .
 8. A multisound reproducing system as recited in claim 6 furthercomprising switching means connected to the outputs of the third and thefourth phase shifting circuits and to the outputs of the means forproducing the third signal and the means for producing the fourth signalfor selectively transmitting the third and the fourth signals or thephase shifted signals from the third and the fourth phase shiftingcircuits to the means for producing the third and the fourth outputs.